Various flat panel displays that can reduce weight and volume with respect to that of a cathode ray tube display have been developed. Examples of flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and a light emitting display (LED).
The LCD displays desired images by controlling light beams emitted from a back light unit using an LCD panel. The LCD panel includes a plurality of liquid crystal cells and a plurality of control switches for switching video signals to be supplied to the respective liquid crystal cells.
As shown in FIG. 1, the related art back light unit includes a lamp 10 generating light, a light guide plate 20 emitting a light source through inner dispersion of the light generated from the lamp 10 and entering the light guide plate 20 through an incident surface 22, a lamp housing 12 arranged to surround the lamp 10 and disposed opposite the incident surface 22 of the light guide plate 20, a reflecting plate 30 arranged below the light guide plate 20, a diffusion sheet 40 arranged above the light guide plate 20 to diffuse the light passing through the light guide plate 20, and a prism sheet 50 controlling a direction of the light passing through the diffusion sheet 40.
Generally, the lamp 10 is a cold cathode fluorescent lamp. The lamp 10 is lit by a lamp driving voltage from an inverter (not shown).
The lamp housing 12 has a reflecting surface therein to reflect the light from the lamp 10 toward the incident surface 22 of the light guide plate 20.
The light guide plate 20 allows the incident light from the lamp 10 to reach a portion away from the lamp 10, and guides the incident light to the diffusion sheet 40.
A reflective pattern is formed at a lower side of the light guide plate 20 to reflect the light from the incident surface 22 at a tilted rear surface of the light guide plate 20 and direct the reflected light to the diffusion sheet 40.
The reflecting plate 30 is arranged below the light guide plate 20 so as to reflect a light emerging through the rear surface of the light guide plate 20 back to the light guide plate 20 so as to reduce light loss. The diffusion sheet 40 diffuses the light passing through the light guide plate 20 and emits the light towards the prism sheet 50. The prism sheet 50 serves to condense the light passing through the diffusion sheet 40.
As shown in FIG. 2, the prism sheet 50 includes a condensing film 52 of polyester (PET) and a plurality of prism peaks 54 formed on the condensing film 52 in a stripe shape.
The prism peaks 54 have first and second tilt surfaces tilted at a predetermined angle. Each of the first and second tilt surfaces is tilted with respect a top surface of the condensing film 52 at an angle of 45°.
The incident light with a predetermined angle θ1 to the prism sheet 50 having a refractive index n1 is refracted by the prism sheet 50 at a predetermined angle θ2 in accordance with the Snell's law of refraction expressed in the following equation 1 the region outside of the prism sheet 50 has a refractive index n2.
                                          n            ⁢                                                  ⁢            1                                n            ⁢                                                  ⁢            2                          =                              sin            ⁢                                                  ⁢            θ            ⁢                                                  ⁢            1                                sin            ⁢                                                  ⁢            θ            ⁢                                                  ⁢            2                                              equation        ⁢                                  ⁢        1            
The light emitted from the lamp 10 proceeds to the diffusion sheet 40 arranged above the light guide plate 20. The diffused light is condensed through the prism sheet 50 and condensed light is finally emitted to the outside.
However, the incident light to the prism sheet 50 can split into three regions, as shown in FIG. 3: viz., a total reflection region, a condensing region, and a side lobe region.
Light ray A of the total reflection region enters the condensing film 52 vertically and is totally internally reflected by the first and second tilt surfaces of the prism peaks 54. The totally reflected light proceeds again to the light guide plate 20. In this way, the light A is condensed by recycling by the reflecting sheet 30.
Light ray B of the condensing region enters the condensing film 52 at an angle and is condensed in such a manner that it is refracted by the lower surface of the prism sheet 50 and one of the first and second tilt surfaces of the prism peaks 54.
Light ray C of the side lobe region enters the condensing film 52 at an angle and is totally internally reflected by one of the first and second tilted surfaces of the prism peaks 54. In this case, light efficiency and viewing angle characteristics are deteriorated.
As shown in FIG. 4 and FIG. 5, a bright region exists at both sides of each of the prism sheet 50 around a symmetrical point due to the side lobe.
As a result, the related art back light unit has some problems due to the structure of the prism sheet 50. That is, viewing angle characteristics are deteriorated by luminance asymmetry in the vertical and horizontal axes (Y and X axes), and condensing efficiency is deteriorated by the side lobe.